Frequency unshifting with power intensification of a whistler wave by a collapsing plasma medium

A source wave is assumed to be present in an unbounded magnetoplasma medium. The parameters are such that the refractive index is greater than one. A wave propagating in the ''whistler mode'' is an example of such a wave. By collapsing the ionization, it is possible to obtain a frequency upshifted wave with power intensification. Two limiting cases of (i) sudden collapse and (ii) slow decay are considered. In either case, it is shown that the final upshifted frequency is the source frequency multiplied by the refractive index. When the source frequency is much less than electron gyrofrequency the refractive index n(w) is quite large and the electric field is intensified by a factor of n(w)/2 for the case of sudden collapse and by a factor of n(w)/root 2 for the case of slow decay. The corresponding intensification factors for the power density are n(w)/4 and n(w)/2. A physical explanation of the results, based on energy balance, is offered. (C) 1996 American Institute of Physics.